1. Field of the Invention
The present invention relates to an image signal processing unit according to which input image signals are replaced by image signals which are suitable for display or printing.
2. Description of the Prior Art
Image display devices, such as television receivers and photocopy machines, are used daily in homes and offices. These image display devices are required to form images which have special qualities to respond to the needs of the viewers, rather than to reproduce images in a natural and exact manner. For example, with a color television receiver in Japan, the skin color is displayed more whitish than the natural color to correspond with the color which is preferred by the viewer. Another example of processing of image data according to perception or preferred taste by the viewer is the emphasis of contour. This is based on the fact that human eyes perceive objects mainly on the basis of contour data. Especially in a display, such as a poster, in which clarity is of prime importance, the edges of the letters are fringed to emphasize the contour.
A prior art example of an image display device for obtaining an image with emphasized contour will be described. An image with emphasized contour may be obtained by the addition of an original image with a contour image. The contour image is considered to be an image consisting of high frequency signal components of the original image. According to an example of a method for extracting high frequency signal components from the original image, an image signal is formed which is the absolute value of the difference between the original image and an image which is slightly shifted from the original image. According to this method, the image signal of the absolute value of the signal obtained by subtracting spatially different signals of the original image is obtained, thus establishing the extraction of the high frequency signal components of the signal of the original image.
In order to obtain an image with emphasized contour by the addition of the contour image and the original image, processings such as optical processing, processing with an electric circuit, and processing with a computer have been performed.
However, with the prior art technique described above, an image with emphasized contour is obtained by emphasizing the luminance or density contrast of the edge of the image. This has led to a disadvantage in that a device which is capable of sufficiently displaying gradation is required.
Devices for displaying images with discrete binary patterns are known, such as plasma panels and liquid crystal displays, in which display cells of the same size are arranged in a matrix form and each cell can take only two values (on and off), and ink-jet printers in which the dot pattern formed by the injected ink can take only two values (on and off). These devices are utilized in image display devices such as panel dislays and facsimiles.
These devices must be able to perform contour emphasis display and half-tone display in order to improve the image quality.
The systematic dither method is known as a conventional method for performing the half-tone display with the discrete binary pattern. According to the systematic dither method, the entire display screen consisting of a dot matrix is divided into submatrices of n.times.n dots, and the respective dots of each submatrix are set at mutually different threshold levels. An example of the systematic dither method is described in U.S. Pat. No. 4,032,978 of Wong.
It is known that the image quality may be particularly improved by combining the contour emphasis processing and the half-tone display processing. However, if the width of the contour is below the submatrix of the dither pattern when the half-tone display processing by the dither method is performed after the contour emphasis processing, the relationship between the lightness, saturation, and hue may be disturbed by the signal conversion in the dither method. This leads to mixture of colors.